Field of the Invention
The present invention relates to a wireless communication system, and more particularly, to a method for performing coverage enhancement, by a UE, using multiple cells, and an apparatus supporting the same.
Related Art
3GPP (3rd Generation Partnership Project) LTE (Long Term Evolution) that is an advancement of UMTS (Universal Mobile Telecommunication System) is being introduced with 3GPP release 8. In 3GPP LTE, OFDMA (orthogonal frequency division multiple access) is used for downlink, and SC-FDMA (single carrier-frequency division multiple access) is used for uplink. The 3GPP LTE adopts MIMO (multiple input multiple output) having maximum four antennas. Recently, a discussion of 3GPP LTE-A (LTE-Advanced) which is the evolution of the 3GPP LTE is in progress.
In recent years, machine-to-machine/Internet of Things (M2M/IoT), which connects all every objects through networks to facilitate obtaining and transmitting necessary information anytime and anywhere, thereby making it possible to offer and use various services, has become a major issue for a next-generation communication market.
While early M2M started with a sensor and an RFID network mainly for local areas, various wired/wireless networks may be used with gradually diversifying purposes and characteristics of applications. Recently, M2M based on a mobile communication network receives growing attention in view of the mobility of objects, a wide range of service areas including not only islands and mountains but also the sea, ease of network management and maintenance, security for reliable data transmission, and guarantee of service quality. Accordingly, with studies on the feasibility of M2M started in 2005, the 3GPP has been conducting a full-scale standardization project under the name “Machine Type Communications (MTC)” since 2008.
The 3GPP regards a machine as an entity that does not require direct human manipulation or intervention and defines MTC as a form of data communication involving one or more of machines. Typical examples of the machine include a smart meter and a vending machine that are equipped with a mobile communication module. Recently, with the introduction of a smart phone that performs communication by automatically connecting to a network, without any user operation or intervention, depending on a user's location or conditions, a mobile terminal having an MTC function is considered as a form of a machine. Also, a gateway-type MTC device connected to an IEEE 802.15 WPAN-based subminiature sensor or RFID is also considered.
In order to meet data rate requirements established by the ITU for IMT-Advanced, transmission and reception using a wide bandwidth is necessary. Therefore, a target bandwidth of 100 MHz, which is much wider than a maximum bandwidth of 20 MHz provided by 3GPP Release 8 and is supportable by LTE-Advanced, is set. However, due to the lack of spectrum resources in a low frequency range, it is not easy to contiguously allocate a 100-MHz bandwidth for a mobile wireless network. Accordingly, carrier aggregation (CA) has been introduced. As the name indicates, CA is a technique for simultaneously using multiple aggregated carriers, in which the multiple carriers may provide and occupy a wide transmission bandwidth of up to 100 MHz at the same time.
Since a cell includes a downlink carrier and an uplink carrier, CA is known as cell aggregation. In CA, a user equipment (UE) may form one basic cell as a PCell and may form up to four auxiliary cells as SCells. A PCell performs basic control, such as RRC connection management, radio bearer management, mobility management, and security management. An SCell controls the amount of traffic by adding or removing radio resources for a UE.